Fiber optical communication has became a new fashion in communication and is expected to replace conventional cables as a major communication transmission medium in the future. In a fiber optical communication system, it is necessary to have a photo detector to detect and accept light signals. An avalanche photodiode ("APD") is a well-known optical detector which plays a role in the fiber optical communication system to accept transmitted light signals.
The avalanche photodiode has been described in several literature. For instances, Capasso. F. et. al. have reported a separate absorption and multiplication avalanche photodiode in Electron Letter, 1984, Vol. 20, no.15, pp635-637. Ravi Kuchibhotlla et. al. described a PIN avalanche photodiode in Journal Lightwave Technology, 1991, Vol. 7, No. 7 pp900-905. However, most of the avalanche photodiode were made by Groups III-V, II-VI materials which present many practical problems. Notably, the preparation cost is too expensive, the materials are not easy to obtain, the preparation processes are too complicated as they involve a molecular beam epitaxy (MBE) or a liquid phase epitaxy (LPE) and the process temperature is too high. It is apparent that there is a need for an alternative APD which can be prepared by a material at a low cost and under a low temperature.
Hong et. al. in IEEE Trans. Electron Devices, 1990, Vol. 37, No.8, pp 1804-1809, have described an amorphous silicon material that may be suitable as an alternative for APD production because of its low material cost, low process temperature and feasibility of being made as a large area device. Hwang et. al. reported that one of the advantages of the amorphous silicon material is that it has no lattice mismatch energy gap so that the APD made with the material can be easily changed to adjust its energy level to absorb different wavelength lights. However, the amorphous silicon material is still unavailable until now. It is therefore an object of the invention to provide for a novel avalanche photodiode made from the amorphous silicon material to replace the conventional APD.
It is a further object of the invention to provide for a simple process for preparation of an avalanche photodiode made from the amorphous silicon material which can be practiced under a low temperature and at a low cost to eliminate the problems of high temperature process and high cost as required for the conventional APD preparation.